Purifying alumina.



*llll l'ltlhlllili ii a ldfi tilhl .Prilllil lli llil I HOYVARD F. CHAPPELL, OF NEW YORK, N. Y.

PURIFYING ALUMINA.

No Drawing.

1 b (17/? 10. mm it may concern:

Be it known that I, Howann F. CHAP- ricLL, a citizen of the United States, residing in the borough of lilanhattan, city, county, and State of New York, have invented certain new and useful Improvements in Puritying Alumina; and IV do hereby declare the following to be a full, clear, and exact 'description of the invention", such as will enable others skilled in the art. to which it up pertains to make and use the same,

The present invention relates to a cheap and eilicient method for removing silica iii-om alumina, and it is based upon the discovery that when the silica-containing alumina. is raised to a sutiiciently high temperature,..-it becomes practically insoluble in hydrofluoric acid, the solubility of the silica remaining practically unaffected; so that, by subject ing the material, after it has been heated to the desired high temperature, to the action of hydrofluoric acid, either in solution or in a gaseous state, thesilica is readily converted into hydrofluosil icic acid or into silicon l'luorid, either of which is readily separable from the alumina.

The invention applicable, for instance, to the treatment. of impure alumina obtained by calcining alunitc until the alumina contained in it is insoluble in Water, and then Washing out the alkali present in the calcined product. The alumina thus obtained, contains silica, which it is desirable to remore in order to permit the alumina to be employed to advantage in the production of aluminum.

For the removal of the silica from alumina obtained from alunite, or from other alumina containing silica wherein the alumina has been rendered insoluble in hydrofluoric acid, by first heating to the temperatures hereinafter referred to, I may proceed, in accordance with the present invention, in various Ways. Thus, I may mix the alu mina with a solution, in Water, of hydrofluoric acid, in excess of the amount required to combine with all of the silica contained in the alumina. This mixture should preferably be stirred-repeatedly and may then be allowed to stand at ordinary atmospheric temperatures, for about 12 hours and until the reaction is complete. The strength ot'the solution employed for the treatment oi alumina obtained, described, from cal- Specification of Letters Patent.

Patented Item Q35, T3 013,.

Application filed April 22, 1913. Serial No. 762,927,

cined alunite may vary from 5% to 48%, but I have found that, there is no advantage (except in shortening somewhat the time required for the operation) in using the higher percentages. I prefer using a 25% solution, the employment of which for the treatment of alumina containing; appw :iinately 6% silica suliicing to reduce the su c; contents to as low as from .01 to .10"); that is, removing substantially all the silica. l'. have found that the amount of: alumina made soluble by this treatment is practically on important, not exceeding say from 2-7}. to 3% of the alumina treated.

Inorder to separate the silica from the mass, at the termination of the treatment with the solution of hydrofluoric acid, the solution may be filtered, and the residue Washed so as to free it as far as possible from hydroiiuosilicic acid present. The residue may then be ignited in the open air, that is to say, heated in a flame at a ten'iperature preferably higl'ler than 700 C, whereupon the silicon iiuorid formed is volatilized and driven 005'. 01', instead of filtering and Washing out the hydrofluosilicic acid from the mass, the entire mixture may be evaporated to dryness and then ignited as described.

As hereinbefore indicated, the hydrofluoric acid may be employed in the gaseous state. This is effected, for instance, by passing the gaseous hydrofluoric acid through the alumina, which has been previously heated to a temper; tore suiiiciently high to prevent the hy droi' uoric acid from upon the aluminato any'important e For the production of the best eiiects, in this treatment with hydrofluoric acid gas thetemperature in the reaction vessel should be above 600 C. and the. reaction should take place in the presence of steam or water vapors; as, for instance, by placing the silica-containing alumina in an iron retort, heated externally to a temperature above 600 C, and either allowing a. solution of hydrofluoric acid gas in Water to drop on the heated alumina, or, by conducting hydrofluoric acid g'cs, together with steam, through the retort.

The temperature to which the alumina should he raised in order to render it pra'ctically insoluo in hydrofluoric acid, in accordance Wu .e present inven n should preferably be as high as 1000 C., and it may to advantage be carried to still higher A degrees, say up to 1500 C.

Under certain conditions, it may not be necessary or desirable to render all of the alumina insoluble. In such cases, the preliminary heating operation may stop short of conversion of more than a portion of the alumina into the insoluble condition, It will be understood, therefore, that my invention includes broadly the feature of rendering alumina insoluble in hydrofluoric acid, by raising it to the temperature required for that purpose, whether such conversion be quantitative or otherwise, so that the silica maybe removed, regardless as to whether or not a'portion of the alumina remains soluble and is afterward converted into 'fluorid' by the action of the hydrofluoric acid.

Having thus described my invent-ion what I claim is:

1. The method of removing s'llicafrom alumina containing the-- same, which comprises first heating it to a temperature suflicient-ly elevated to render alumina practically insoluble in hydrofluoric acid, then sub ecting it to the action of hydrofluoric acid and removing the product of the reaction;substantially as described.

2. The method of removing silica from alumina containing the same, which .comprises first, heating it to a temperature sufli-.

eiently elevated to render alumina insoluble in hydrofluoric acid, then subjecting it to the action of hydrofluoric acid and removing by ignition the product of the reaction; substantially as described.

3. The method of removing silica from alumina containing the same, which comprises first heating it to a temperature sufiiciently elevated to render alumina insoluble in hydrofluoric acid, then subjecting it to the action of hydrofluoric acid and removing by ignition, assisted by preliminary washing, the product of the reaction; substantially as described.

4. The method of removing silica from alumina, which comprises first heating it i alumina, which c miprises first heating it to a temperature sufliciently elevated to render alumina insoluble in hydrofluoric acid, then subjecting it to the action of hydrofluoric acid in the presence of water and at a temperature above 600 C. and removing the product of the reaction; substantially as described.

In testimony whereof I afiix my signature, in presence of two witnesses. v

HOlVARD F. CHAPPELL. Witnesses:

M. A. BlLL,

JOHN C. PENNIE. 

